This invention relates to data transmission networks and more specifically relates to a novel data transmission network for communicating signals between apparatus being monitored and controlled and remote controlling equipment.
It is frequently desirable to employ remotely located monitoring and control circuitry for monitoring the operation of equipment and controlling its operation in response to monitored data. For example, in a high voltage electric power transmission system it is desirable to monitor the voltage and current of the system and to operate circuit switching equipment or other similar equipment in response to the measured data. All of the circuitry for handling the monitored signals and for producing appropriate control signals is preferably remotely located where it is accessible at ground level to operators and available for maintenance and the like.
It is possible to employ a first set of control wires extending from the line monitoring devices such as current transformers to the remote control circuits, and to employ a second set of control wires extending from the remote control circuits at ground level to the circuit switching devices at high voltage. These two separate circuits, however, are subject to electromagnetic interference. Moreover, if they are closely coupled to one another, they can interfere with the operation of one another. It is also known to produce monitoring signals as a modulated light beam and carry the monitor signals from the equipment being monitored to the ground location over a first optical wave guide. Control signals produced at ground can be converted to modulated light signals and carried to controls for the equipment being monitored by a second optical wave guide. While this avoids possible electromagnetic interference, it requires two duplicate and separate optical paths.
The use of such fiber optic systems in data transmission networks is described in "THE USE OF FIBER OPTICS FOR COMMUNICATIONS, MEASUREMENT AND CONTROL WITHIN HIGH VOLTAGE SUBSTATIONS" by D. C. Erickson, IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, VOL. PAS-99, NO. 3, (1980), pp. 1057 et seq.